Arachidonic acid (ARA) is a long chain polyunsaturated fatty acid (PUFA) of the omega-6 class (5, 8, 11, 14-eicosatetraenoic acid, i.e., 20:4). ARA is the most abundant C.sub.20 PUFA in the human body. It is particularly prevalent in organ, muscle and blood tissues, serving a major role as a structural lipid associated predominantly with phospholipids in blood, liver, muscle and other major organ systems. In addition to its primary role as a structural lipid, ARA also is the direct precursor for a number of circulating eieosenoids such as prostaglandin E.sub.2 (PGE.sub.2), prostacyclin I.sub.2 (PGI.sub.2), thromboxane A.sub.2 (T.sub.x A.sub.2), and leukotirenes B.sub.4 (LTB.sub.4) and C.sub.4 (LTC.sub.4). These eicosenoids exhibit regulatory effects on lipoprotein metabolism, blood theology, vascular tone, leucocyte function and platelet activation.
Despite its importance to human metabolism, ARA cannot be synthesized in humans de novo. ARA is synthesized by the elongation and desaturation of linoleic acid (LOA), an essential fatty acid. This process requires the presence of the enzyme .DELTA.6-desaturase, an enzyme present in the human body in low levels, Burre et al., Lipids, 25:354-356 (1990). Accordingly, most ARA must be provided in the diet, and this is especially important during times of very rapid body growth, such as infancy.
During the first year of its life, an infant can double or triple its weight. Consequently, elevated levels of dietary ARA are required. To satisfy this increased demand, human breast milk contains high levels of ARA. Sanders et al., Am. J. Clin. Nutr., 31:805-813 (1978). ARA is the most prevalent C.sub.20 PUFA in breast milk. Of those mothers, especially vegetarians, who do breast-feed their infants, many would benefit from additional dietary ARA. However, many mothers do not breast feed their infants, or do not breast feed for the entire period of rapid infant growth, choosing instead to utilize an infant formula.
No commercial infant formulas known to Applicant contain ARA in triglyceride form. U.S. Pat. No. 4,670,285 (Clandinin et al.), incorporated herein by reference, discloses the infant's requirement for fatty acids including ARA. To provide these fatty acids, Clandinin et al. suggest a blend of egg yolk, fish oil or red blood cell phospholipids and vegetable oils as the fat component of a proposed infant formula. However, fish oil contain high quantities of eicosapentaneoic acid (EPA). EPA is known to depress ARA synthesis in infants. Carlson, et al., INFORM., 1:306 (1990). Thus, it would be desirable to be able to provide ARA without also providing additional EPA. Furthermore, egg yolks contain a relatively low concentration of ARA, such that Clandinin et al.'s mixture is not economically viable.
Because ARA is present in animal, but not vegetable, oils, its production in commercial quantities has remained a desirable, but elusive, goal. Shinmen, et al., Microbiol. Biotech. 31:11-16 (1989), have reported the production of ARA by an isolated fungus, Mortierella alpina, using conventional stirred tank fermentation. (See also Japanese Patent 1,215,245 to Shinmen et al.). After culturing, the organisms are harvested, dried and their lipids extracted from the fungal biomass with an organic solvent and the lipids chemically (covalently) modified. For example, the lipid mixture is hydrolyzed or converted to ethyl esters and then combined with cyclodextrin prior to use as a dietary supplement. Shinmen et al. do not disclose or suggest the administration of unmodified microbial oils.
Porphyridium cruentum, a red microalgae, can be grown in ponds in large quantities and has a lipid content which can contain up to 40% ARA. Ahem, et al. Biotech. Bioeng. 25:1057-1070 (1983). Unfortunately, the ARA is primarily associated with galactolipids, a complex polar lipid not present in breast milk. Thus, not only is the total usable ARA produced a fraction of one percent of the biomass, but the form of the ARA is not suitable for use as an additive to infant formula without further modification.
U.S. Pat. No. 4,870,011 (Suzuki et al.) discloses a method for obtaining lipids such as .gamma.-linolenic acid from fungi of the genus Mortierella. The .gamma.-linolenic acid is purified from the mixture of lipids contained in the fungi.
DE 3603000A1 (Milupa) discloses a highly polyunsaturated acid fat mixture and its use as the fat component of an infant formula. The fat mixture has a high content of ARA and docosahexanoic (DHA) acids in a ratio of 2.5:1 respectively, as well as a high content of cholesterol. Sources of the fatty acids are listed as being certain types of macroalgae, fish oils, organ fats from beef and pork or highly refined egg yolk off. A source of the DHA and ARA is said to be macroalgae of the phaecophyte and rhodophyte types. There is no suggestion to use any microbes as a source of oil. Algal and fish oils also typically include EPA which depresses ARA synthesis in vivo. Additionally, highly refined egg yolk oil is not an economical source of ARA. Moreover, there is no disclosure therein of an ARA-concentrated additive for supplementing pre-existing infant formula.
Accordingly, there remains a need for an economical, commercially feasible method of producing ARA, preferably without concomitant production of EPA. It is an object of the present invention to satisfy that need.
It is a further object of the invention to provide an additive, and a source for that additive, for use in an infant formula such that the ARA levels in the formula approximate those levels in human breast milk.
It is an additional object of this invention to provide an ARA-containing fungal oil for use in enteral, parenteral or dermal products.